/**
 * Definition for singly-linked list.
 **/
  class ListNode {
      int val;
      ListNode next;
     ListNode() {}
    ListNode(int val) { this.val = val; }
      ListNode(int val, ListNode next) { this.val = val; this.next = next; }
  }

class Solution {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        if(list1 == null){
            return list2;
        }else if(list2 == null){
            return list1;
        }else if(list1.val < list2.val){
            list1.next = mergeTwoLists(list1.next,list2);
            return list1;
        }else{
            list2.next = mergeTwoLists(list2.next,list1);
            return list2;
        }

    }
}